The child - now four years old, was born prematurely in a Mississippi clinic in 2010 to an HIV-infected mother. After being given treatment for 18 months, the child then went for more than two years without antiretroviral medicines and during that time blood samples revealed undetectable HIV levels.

"Certainly, this is a disappointing turn of events for this young child, the medical staff involved in the child's care and the HIV/AIDS research community," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infections Diseases.